Modeling And Numerical Simulation Of Fe-Based Powder Forming In Thermal-electrical-mechanical Coupled Fields

As an important green manufacturing technique, powder metallurgy (P/M) has many compelling advantages over the traditional manufacturing processes, such as the cost and material saving, the high production rates and the near net-shape capacity. P/M technique can control the material properties of microstructure, density and alloy compositions et al. The large demand of P/M parts promotes the innovation of P/M techniques and gives birth to a lot of advanced technologies including hot isostatic pressing (HIP), spray forming, spark plasma sintering, and so on. The finite element method (FEM) is generally applied to study and optimize the forming process of powders, which has becomed a hot spot in P/M research field.The numerical modeling and FEM simulation of the compaction forming and SPS processes are presented in this paper for the metal powder material. The corresponding experimental discussions and analyses are conducted to validate the accuracy of the adopted theoretical model.The thermal elastoplastic constitutive model in the incremental form is deduced on the basis of the ellipsoidal yield criterion and the plasticity flowing rule. The relationship of the elastoplastic stress and strain in the complex loading and unloading situations is calculated by the returning mapping algorithm. Different yield models are compared in the plane of hydrostatic stress and Mises equivalent stress. It is considered that the ratio of Mises equivalent stress and hydrostatic stress can be used to analyze the error of the ellipsoidal yield criterion qualitatively which is derived from the shear yielding.The user subroutines of the constitutive model are integrated with the commercial FEM software Marc, which is applicable to the complex 3D simulation of the forming processes for powder materials. The simulative results show that this new model gives a better description on the densification behaviors of powders in the cold compression processes. The increase of the flowing stress can also be calculated more precisely. Because of the more rational material parameters, the influences of the deviator of the stress tensor and hydrostatic stress are weighed more veritably. As a result, the present model fits better with the experimental load-displacement curves in the later stage of the compaction process.A perforation phenomenon which occurs in the SPS process of the metal powder material is discussed. It can be concluded from the analyses of microstructure that the unexpected local densification behavior could affect the distributions of electrical potential, stress and temperature significantly, which demonstrates the importance of the density gradient to the SPS FEM study.Based on the discussions of these SPS experimental phenomena, this paper proposed a simulation scheme of the multi-physical fields, which realizes the coupling of temperature field, electric field, stress field and density field. The simulation shows that the increased outer pressure helps to decrease the sintering temperature and restrain the intrinsic radial temperature difference effectively through affecting the variational compact/die contact thermal resistance within the displacement field. Moreover, the comprehensive actions of stress promote the densification process of the colder regions in the interior of the powder compact. And the reliability of the coupled multi-physical-field FEM model is confirmed by the corresponding experiments.